Threaded joints are typically used as a means for connecting oil well pipes. In recent years, there has been increasing application of threaded joints to steel pipes which have conventionally been connected by welding, such as riser pipes and line pipes.
Up to now, a standard threaded joint was one meeting API (American Petroleum Institute) standards, but in recent years, the environments of excavation and production of crude oil and natural gas are becoming more severe, so there is increasing use of special threaded joints referred to as premium joints.
FIGS. 1(a) and 1(b) are a schematic cross sectional view of one example of a premium joint and an enlarged view of a portion thereof. Normally, a premium joint has tapered threads 12 and 22, a metal seal portion 4, and a torque shoulder portion 5. The side on which a tapered male thread 12 is provided is referred to as a pin 11, and the side on which a female thread 22 is provided is referred to as a box 21.
The arrangement and combination of the tapered threads, metal seal portions, and torque shoulder portions, the number of each which are provided, and the like vary with the use of the joint. For example, the metal seal portions and the torque shoulder portions are installed at positions A, B, and C shown in FIG. 2(a), i.e., at position A which is on the outer side of the threads, at position B which is between the threads, and at position C which is inwards of the threads.
For example, there are cases in which a metal seal portion and a torque shoulder portion are disposed on the inner side of the joint (FIG. 2(b)), those in which a metal seal portion is disposed on the inner side of the joint and a torque shoulder portion is disposed on the inner side and outer side (FIG. 2(c)), those in which a metal seal portion is provided on the inner side and outer side and a torque shoulder portion is provided at the middle (FIG. 2(d)), and those in which a metal seal portion is provided at the middle and on the inner side and a torque shoulder portion is provided on the outer side (FIG. 2(e)).
As shown in FIGS. 3(a) and 3(b), joints include coupling types (see FIG. 3(a)) in which steel pipes having a pin (a male threaded member) on both ends are connected by a coupling having a box (a female threaded member) provided at both ends of a short pipe, and integral type (see FIG. 3(b)) in which steel pipes having a pin provided on one end and a box provided at the opposite end are directly connected to each other.
Next, the components of a joint will be described in detail using the most typical structure which is shown in FIG. 1 and FIG. 2(b), as an example.
Namely, most threads used in a premium joint are ones resembling a trapezoidal thread referred to as API buttress threads shown in FIGS. 4(a) and 4(b). In this thread, the flanks, which strongly contact at the time of completing make-up, are referred to as load flanks 16, and the flanks on the other side are referred to as stabbing flanks 17 and 23. With a usual buttress thread, there is contact at the thread root surface 14 and a gap at the thread crest surface 18.
Gas tightness, which is one of the important properties of a premium joint, is exhibited by interfitting the metal seal portions with a suitable amount of interference determined by design.
Here, interference refers to a press fit of the sealing surfaces and it is a value given by (diameter of the metal seal portion of the box) minus (diameter of the metal seal portion of the pin).
In order to obtain a strong threaded connection, interference is also provided in the tapered thread portions. In the case of the above-described buttress thread, there is strong contact at the thread root due to the interference of the thread portions.
In this case, interference is the value given by (diameter of the thread portion of the box) minus (diameter of the thread portion of the pin).
Here, the torque shoulder portions function as stoppers for providing a suitable fit between the metal seal portions.
A premium joint can exhibit its properties to a maximum extent by abutting the torque shoulder portions within the range of elastic deformation and completing make-up in a state in which a make-up force in the axial direction is generated within the joint.
If the torque shoulder portions do not abut, a make-up force in the axial direction is not generated, and the amount of tightening becomes inadequate, whereas if the torque shoulder portions abut each other too strongly, the torque shoulder portions end up undergoing plastic deformation, and the sealing properties of the adjoining metal seal portions end up decreasing.
In the past, vertical wells were predominant, so the fundamental properties required of premium joints were the ability to withstand a tensile load due to gravity and an ability to seal against internal and external fluids.
However, in recent years, horizontal wells and sloping wells are increasing, and threaded joints have also come to be used for connecting riser pipes for undersea oil fields.
FIGS. 5(a) and 5(b) schematically explain these states. As shown in FIG. 5(a), an oil well 33 on the sea bottom is connected by a vertically-descending riser pipes 35 to a rig 34 on the surface of the sea, so a large compressive force acts on the riser pipe. In addition, recently, as shown in FIG. 5(b), there are cases in which oil well pipes 31 extend in the horizontal direction or in which oil well pipes 32 are bent. When installing oil well pipes underground in a horizontal well or the like, in order to pass a straight oil well pipe through a curved portion within the ground, the oil well pipe is inserted while being rotated, and at this time, a large compressive force acts on joint portions.
Therefore, the ability to withstand compression is a new property which has come to be demanded of a premium joint.
In addition, in recent years, due to the deepening of oil wells or excavation and production in locations having poor soil properties, there are cases in which oil well pipes undergo large external pressures due to the load from fluids other than products (crude oil and natural gas) outside the well bore or contraction and collapse of the well.
Under such conditions, the ability to seal against external pressure so as to prevent the penetration of high pressure fluids from the exterior has come to be a newly demanded property.
Oil wells typically have a depth from around 3,000 meters to 6,000 meters, and oil well pipes having a length of around 8-10 meters are inserted into a well bore from atop the ground or from atop a rig on the surface of the sea while connected by the above-described threaded joints. Namely, in order to install oil well pipes underground in a single oil well, make-up of threaded joints is carried out approximately 300 to 750 times.
Once underground installation is commenced, it is continued night and day until installation is completed, and it requires approximately two to three days. If make-up of a joint requires one minute extra, for example, overall installation ends up requiring an extra 6 to 12 hours. This results in additional labor costs for field operations and costs for equipment rental, leading to an increase in costs.
Therefore, ease of make-up of a threaded joint is an extremely important property.
Resistance to Compression
Among threads having the object of increasing resistance to compression, there is a thread in which the load flanks and the stabbing flanks of a trapezoidal thread are made to contact and gaps are provided at the thread root surface and the thread crest surface (referred to below as a rugged thread).
FIG. 6(a) is a schematic explanatory view of the cross-sectional shape of a rugged thread 22, and FIG. 6(b) is an explanatory view showing the distribution of forces thereon.
This type of rugged thread 25 is already disclosed in Patent Document 1 and Patent Document 2. In the former, the load flank angle is limited to −20° to 0E, and in the latter, it is limited to 0° to +3°.
In a rugged thread 25, the stabbing flanks 17 of the threads 12 and 22 contact, so a compressive load can be borne by the stabbing flanks 17 of the thread, and it has extremely high resistance to compressive force. On the other hand, there is contact at the load flanks 16 and the stabbing flanks 17 of the threads 12 and 22, so the tolerance of the thread 19 has an extremely large influence on the torque generated by the thread.
With a premium joint, make-up is normally controlled based on the torque. Namely, the torque reaction is monitored during make-up, and make-up is completed when a prescribed make-up torque is reached. However, with a rugged thread, the variation in torque due to thread tolerances is extremely large, so it is difficult to set a suitable make-up torque.
A suitable torque indicates a make-up torque which can make the shoulders abut within the range of elastic deformation for all products made to manufacturing tolerances. However, in extreme cases, the torque at the start of shoulder contact (referred to as the shouldering torque) of a product within tolerances exceeds the torque at which the shoulders undergo plastic deformation (referred to below as overtorque) for another product which is within tolerances, and a suitable torque no longer exists.
The above-described situation (generally referred to as the high shouldering problem) is unavoidable with present-day manufacturing control techniques (allowable manufacturing tolerances) particularly with respect to small-diameter rugged threads having an outer diameter of 5 inches or less.
None of the above-described prior art documents has any description with respect to this high shouldering problem, so they do not disclose a method for solving it.
Ability to Seal Against External Pressure
With the object of improving the ability to seal against external pressure, in Patent Document 3, for example, a large portion of a pin member is increased in thickness by swaging (reduction of the inner diameter of the pipe end).
In addition, in Patent Document 4, the resistance to deformation of the end of a pin is increased as much as possible by providing a male thread as close as possible to the torque shoulder of the end of the pin.
In any of the above-described prior art, fundamentally, the resistance to deformation in response to external pressure (resistance to contraction) is increased by increasing the wall thickness of the pin member or of the connecting portion between the torque shoulder at the end of the pin and the thread portion to reduce the decrease in diameter, and a gap is prevented from forming at a metal seal.
Any of this prior art has the effect of increasing the ability to seal against external pressure.
However, in Patent Document 3, it is necessary to perform swaging over a considerable length of the pipe end (probably around 100-200 mm), so a horizontal rolling mill of extremely large power is required, and the swaging die rapidly deteriorates, so there is concern of manufacturing costs becoming quite high. In addition, the inner diameter of a joint is reduced by considerable ratio, so turbulence when internal fluids pass through the joint portion becomes severe and there is a concern of its causing erosion. In addition, in the case of an oil well pipe, the size of pipe which can be inserted into its interior decreases, so the string design ends up being having extremely poor efficiency.
In Patent Document 4, there is the effect that the decrease in the interference of the metal seal portion can be restrained to a certain extent by the amount that the decrease in diameter of the connecting portion between the torque shoulder at the end of the pin and the thread portion is reduced, but compared to the technique in Patent Document 3 in which the thickness of the pin lip is directly increased, its effect is limited.
Ease of Make-up
Prior art with the object of increasing ease of make-up includes Patent Document 5 and Patent Document 6.
This prior art has the object of increasing make-up speed and stabbing properties.
Stabbing properties refer to how smoothly insertion can be carried out to a state in which thread engagement is started at the time of inserting the pin of a male threaded member into the box of a female threaded member. The more easily a pin can be stably inserted to the rearmost portion of a box even when there is a large deviation between the axis of the pin and the box or of the angles therebetween, the better are the stabbing properties of a joint.
Make-up speed is the number of rotations in which a joint can be tightened. The larger the thread taper, the lower the thread height, the greater the thread pitch, and the larger the number of thread ridges, the smaller the number of rotations required for make-up.
The above-described prior art aims at improving stabbing properties and make-up speed primarily by employing a multiple-start thread or by adjusting the slope of the thread crest surface or the thread height.
However, the above-described prior art only improves the ease of make-up, and it cannot be said to improve resistance to compression.    Patent Document 1
Japanese Patent Number 2705505    Patent Document 2
Japanese Published Unexamined Patent Application Hei 11-294650    Patent Document 3
Japanese Published Unexamined Patent Application 2002-22070    Patent Document 4
Japanese Published Unexamined Patent Application 2001-317668    Patent Document 5
Japanese Published Unexamined Patent Application Hei 11-201344    Patent Document 6
Japanese Published Unexamined Patent Application Hei 11-223284